Method and apparatus for gasifying carbonaceous material

ABSTRACT

Method and apparatus for gasifying carbonaceous material, in which (a) product gas and ash, residual carbon a gasified tar compounds entrained therewith are discharged from a gasifying reactor to a product gas channel and cooled in a gas cooler, whereby tar compounds condense in a liquid state and tend to stick on heat surfaces; (b) solids containing ash particles and residual carbon separated from the gasification system, preferably from its product gas, are supplied to an ash reactor, in which the residual carbon reacts with oxygen and ash particles and exhaust gas is generated; and (c) ash particles are supplied to the gas cooler or upstream from the gas cooler, whereby the ash content entrained with the product gas increases and the sticking of condensing tar compounds on the heat surfaces decreases.

The present invention relates to a method and an apparatus in accordancewith the preambles of the independent claims.

Thus the present invention relates to a method of gasifying carbonaceousmaterial, in which method carbonaceous material is gasified to productgas in a gasification reactor of a gasification system; said product gasand the ash entrained therewith, residual carbon and gaseous tarcompounds are discharged from the gasification reactor to a product gaschannel; and product gas is cooled in a gas cooler, whereby tarcompounds condense in a liquid form, which tend to stick on surfaces.Moreover, solid material containing ash particles and residual carbon isseparated from the gasification system and said solid material separatedfrom the gasification system is guided to an ash reactor.Oxygen-containing gas is supplied thereto, whereby residual carbonreacts with oxygen generating ash particles and exhaust gas.

The invention also relates to an apparatus for gasifying carbonaceousmaterial. Said apparatus comprises a gasification system including agasification reactor, a product gas channel connected to thegasification reactor, a gas cooler arranged in the product gas channeland means for separating solid material containing ash particles andresidual carbon from the gasification system; an ash reactor havingmeans for treating residual carbon from the ash with oxygen; and meansfor supplying solid material separated from the gasification system tothe ash reactor.

When gasifying carbonaceous fuel, air and/or oxygen as well as steam aresupplied to the gasification reactor, whereby the aim is to generateproduct gas mainly consisting of carbon monoxide CO and hydrogen H₂.

To bring about endothermic reactions generating carbon monoxide andhydrogen, heat must be either released from the fuel by means of partialcombustion or it must be brought to the gasification reactor in the formof external heat exchange medium. When the gasification is not complete,part of the carbon in the fuel exits with the product gas asnon-gasified char. The gasification temperature, especially withfluidized bed gasifiers, is often relatively low, e.g. 500-1000° C.,whereby the non-gasified carbon may significantly reduce the gasproduction level of the gasifier.

The product gas leaving the gasification reactor generally contains ashparticles, which must be removed, for example, by a particle filterbefore further processing the product gas. Since the particle filtersfor gas operating at a high temperature are expensive and are prone tobe damaged, the product gas is usually cooled prior to filtering.Especially, when gasifying waste material and biomass, significantamounts of tar compounds may generate, which are gaseous at thegasification temperature, but condense at lower temperatures to stickydrops and even to solid particles, which may, for example, form depositson the heat exchange surfaces of the gas cooler and on the filter, whichagain are difficult to be removed. Thus, the tar compounds decrease theheat exchange capacity of the heat exchange surfaces and clog filteringelements of the filter increasing the pressure loss caused by thefilter.

U.S. Pat. No. 5,658,359 discloses a method, in which heat exchangesurfaces of a gas cooler in a fluidized bed gasifier are mechanicallycleaned of deposits by passing bed sand, limestone or material separatedfrom the product gas by a particle filter downstream of the gas coolerto the heat exchange surfaces.

U.S. Pat. No. 4,613,344 discloses a method, in which sticking ofimpurities of the product gas is prevented by quickly cooling the gasthrough critical temperature zones. Cooling of gas is accelerated byadding inert material, e.g. aluminiumoxide Al₂O₃ to the product gas inthe gas cooler, which material is separated from the product gasdownstream of the gas cooler by a centrifugal separator, cooled in aheat exchanger fluidized by product gas and recirculated to the productgas.

Solid material separated from the product gas, e.g. by a particlefilter, may contain in addition to ash also a considerable amount ofchar. The fly ash and bottom ash of the gasification reactor may alsocontain PAH compounds and other carbon compounds harmful to theenvironment. Thus, the ash removed from the gasification system mustusually be post-treated before it may be gathered to public landfillsites or be utilized as raw material, for example, in the industry or inthe agriculture.

WO publication no. WO 00/43468 discloses a method, in which carbonaceoussolid material collected from the product gas filter of a fluidized bedgasifier is oxidized in another fluidized bed reactor and theoxygen-containing gas discharging from the reactor is guided to thefluidized bed gasifier to act as secondary gasification gas.

U.S. Pat. No. 4,347,064 discloses a method in which partially gasifiedmaterial collected from the separators of a circulating fluidized bedgasifier is brought to the final gasification in another gasifier, theproduct gas of which is supplied to act as fluidization gas in acirculating fluidized bed gasifier.

The object of the present invention is to provide a method and anapparatus, by means of which usability of the gasification system forcarbonaceous material is improved.

It is especially an object of the present invention to provide a simplemethod and apparatus for avoiding problems caused by tar compoundsentrained with the product gas.

To solve these problems a method is provided, the characteristicfeatures of which are disclosed in the characterizing part of theindependent method claim. Thus it is a characteristic feature of themethod in accordance with the present invention that ash particles areguided from the ash reactor along a conveying duct to the gas cooler orupstream of the gas cooler, whereby the ash content of the product gasincreases and the sticking of condensing tar compounds onto the heatsurfaces of the gas cooler decreases.

Ash particles are preferably supplied from the ash reactor to the upperpart of the gasification reactor and most preferably directly to theproduct gas channel, whereby the ash particles prevent as effeciently aspossible to the sticking of tar compounds.

To solve the above mentioned problems of the prior art an apparatus isalso provided, the characterizing features of which are disclosed in thecharacterizing part of the independent apparatus claim. Thus it ischaracteristic of the apparatus in accordance with the present inventionthat the apparatus comprises means for guiding ash particles treated inan ash reactor from the ash reactor to a gas cooler or upstream of thegas cooler.

The apparatus preferably comprises means for guiding ash particlestreated in the ash reactor from the ash reactor to the upper part of thegasification reactor and most preferably means for guiding them from theash reactor directly to a product gas channel.

The gasification reactor of a gasification system according to apreferred embodiment of the present invention is a fluidized bedgasifier, but it may also be some other kind of a gasifier, for example,a fixed-bed gasifier or a dust gasifier. The fluidized bed gasifier mayeither be a circulating fluidized bed gasifier or a bubbling bedgasifier. The gasifier may operate, for example, at a temperature of400-1100° C. According to a preferred embodiment, the gasifier operatesat a temperature of about 600-1000° C. and according to a most preferredembodiment at a temperature of about 800-950° C.

Generally, solid material containing ash particles and residual carbonis separated from the gasification system both as so called bottom ashfrom the bottom of the gasifier and as so called fly ash from theproduct gas. Solid material may also be separated from the hot cycle ofthe circulating fluidized bed gasifier. According to the presentinvention it is possible to guide flows of solid material in any of theabove mentioned ways to the ash reactor.

According to a most preferred embodiment of the invention, impuritiesare separated from the product gas in a filter, whereby filtered productgas as well as solid material containing residual carbon and ashseparated from the product gas are obtained. Said solid material isguided to the ash reactor. The average particle size of the solidmaterial separated in the filter is relatively small, whereby it formsin the ash reactor ash having a large surface areea for the tars tocondense on. When the surface area is large, the thickness of thecondensed tar on the surface of the ash particles remains small,wherefor the particles are not sticky and they do not stick on the heatsurfaces or on each other.

In an ash reactor in accordance with the present invention it ispossible to let the residual carbon react with oxygen, whereby carbon iseither combusted or completely gasified. In combustion carbon reactswith oxygen and generates carbon dioxide CO₂ and flue gas containingoxygen. In gasification, carbon reacts with gasification gas containingonly some oxygen, whereby at least carbon monoxide CO is generated.

Different types of reactors may be used as ash reactors, but accordingto a preferred embodiment of the invention the ash reactor is afluidized bed reactor, which may either be a circulating fluidized bedreactor or a bubbling bed reactor.

Ash may be conveyed from the ash reactor along the conveying duct to thegasification system, for example, pneumatically. Since the particle sizeof the solid material arriving in the ash reactor from the filter issmall, typically less than 200 μm, ash from the ash reactor utilizingthe fluidized bed principle is entrained with the fluidizing gas out ofthe reactor. According to a preferred embodiment of the invention,exhaust gas from the fluidized bed reactor and ash particles entrainedtherewith are guided to the gasification system, preferably to theproduct gas channel thereof.

It is possible to separate the coarsest portion of the ash particlesentrained with the exhaust gas being guided to the gasification system,for example, by a centrifugal separator, whereby only the finest portionof the ash particles is guided with the exhaust gas to the gasificationsystem. Part of the coarsest ash separated from the exhaust gas may bereturned to the ash reactor and the rest may be removed to an ashhopper, for example, by means of a cooled screw conveyor. The separationefficiency of the exhaust gas separator of the ash reactor shall bechosen in such a way that a sufficient portion, preferably the majorityof the ash entrained with the exhaust gas remains unseparated and isentrained with the exhaust gas to the product gas channel.

A fluidized bed reactor acting as an ash reactor can operate, forexample, at a temperature of about 700 to 950° C. Preferably, the ashreactor operates at a temperature of about 850° C. For example, heatexchange surfaces may be arranged inside the reactor to control thetemperature of the ash reactor. According to a preferred embodiment ofthe present invention, the temperature of the ash reactor is adjusted bymeans of a gas cooler in the product channel. Thus, the energy beingreleased in the ash reactor may be utilized in a simple manner, forexample, to form steam needed in the gasification reactor.

According to a preferred embodiment of the present invention, ashparticles being guided to the gasification system, especially to theproduct gas channel thereof, from the ash reactor along a conveying ductare cooled already before being introduced to the product gas channel byutilizing a heat exchanger arranged to the conveying duct. Thereby, thetendency of tar compounds in the product gas to condense on ashparticles being returned from the ash reactor increases and the stickingof tar compounds on the heat surfaces of the gas cooler in the productgas duct decreases.

A gas cooler in the product gas duct is used for cooling product gas andalso ash entrained with the gas, preferably to a temperature of about200-350° C. The cooled ash both from the ash reactor and from thegasification reactor is separated from the product gas by means of aparticle filter and is guided to the ash reactor. Since the temperatureof the ash arriving from the filter is lower that that of the ashreactor, the temperature of the reactor may be adjusted by changing theamount of ash circulating through the separator, cooler and filter backto the ash reactor. The amount of the circulating ash may be adjusted bychanging the proportion of the ash being removed from the system in theash being separated by a separator of the ash reactor. When removal ofthe ash being separated is decreased or when it is temporarily stoppedcompletely, the amount of ash in the ash reactor and in thereactor-separator-cooler-filter-reactor cycle increases and thetemperature of the ash reactor decreases. Correspondingly, when removalof separated ash is increased, the amount of the ash in the reactor andin the cycle decreases and the temperature of the reactor increases.

It is a characteristic feature of the present invention that the ashreactor generates ash material, which may advantageously be supplied tothe product gas and thus problems caused by tar compounds in the productgas may be avoided. When the proportion of the ash in the impuritiesentrained with the product gas increases, the proportion of the tarcompounds respectively decreases and sticking of the impurities on thesurfaces decreases. The ash content in the product gas is in the area ofthe ash coolers and the filter preferably at least 100 g/m³. The ashmaterial coming from the ash reactor is inert and the average particlesize thereof is small, wherefor it is especially advantageous fordecreasing problems caused by the tar compounds.

When ash separated from the gasification system is recirculated from theash reactor through the gas cooler and the particle filter for theproduct gas, it is possible to increase the average retention time ofthe ash in the ash reactor. Thus, the residual carbon of the ash mayeither be gasified or combusted almost completely. At the same timehydrocarbon compounds of the solid material removed from the filter,which are harmful to the environment, are effeciently decomposed. Byutilizing the present invention, the efficiency of the plant is thusincreased and at the same time the applicability of the material to beremoved from the reactor is improved, for example, as raw material inthe industry, or alternatively ash may be collected to landfill siteswithout environmental problems.

The invention is discussed below, by way of example, with reference tothe accompanying drawings, in which

FIG. 1 schematically illustrates an apparatus in accordance with apreferred embodiment of the present invention; and

FIG. 2 schematically illustrates an apparatus in accordance with asecond preferred embodiment of the present invention.

A gasification reactor 10 is disclosed in FIG. 1 as a circulatingfluidized bed gasifier, but it might also be of another type of areactor suitable for gasifying fuel containing carbonaceous material.Material to be gasified, inert bed material, such as sand, and, ifnecessary, also sorbent, for example, lime stone are supplied by meansof feeding means 12.

By means of feeding means 14 for fluidizing gas, gasification gas actingas fluidizing gas is introduced to the bottom of the gasifier. Thegasification gas may be air and/or oxygen and possibly steam. Secondarygasification gas may be supplied to the fluidized bed of the gasifier bymeans of means 16. The fluidizing gases and the product gases generatedin the reactor entrain therewith in the circulating fluidized bedreactor solid particles to the upper part of the reactor 10. In saidupper part of the reactor a portion of the solid material exits with theproduct gas through an outlet opening 18 to a particle separator 20. Themajority of the solid material entrained with the product gas isseparated in the particle separator 20 from the product gas and isreturned to the lower part of the reactor 10 by means of a return duct22.

To release the energy required for endothermic gasification reactionspartial combustion of fuel takes place in the lower part of thegasifier. The gasification in a fluidized bed gasifier typically takesplace within a temperature range of 600-1100° C., for example, at atemperature of 850° C. The lower part of the gasifier is provided withmeans for removing bottom ash, said means possibly comprising, forexample, a cooled screw conveyor 24.

The product gas exiting through an outlet opening 26 of the particleseparator 20 still contains impurities containing fine ash, ungasifiedresidual carbon, tar compounds and other carbon compounds, among whichthere may also be compounds harmful to the environment. Subsequent tothe separator 20 the gas flow and the impurities thereof are guided to agas cooler 30 in a product gas channel 28. The temperature of theproduct gas is decreased in the gas cooler 30 to a temperature, forexample, of about 200-350° C., required by a particle filter 32 arrangedin the latter part of the product channel. The tar compounds entrainedwith the product gas, which are gaseous at the temperature of thegasification reactor, condense in the gas cooler 30 to small drops,which tend to stick on the heat exchange surfaces of the gas cooler andon the following surfaces downstream thereof.

The product gases are supplied from the gas cooler 30 to the particlefilter 32, which very efficiently removes all non-gaseous impuritiesfrom the product gas. The cleaned product gas is guided from theparticle filter 32 through an outlet channel 34 to combustion of productgas or to further processing, which may be, for example, reprocessingfor a chemical process.

The solid material separated by the particle separator 32 is guided bymeans of an outlet pipe 36 to an ash reactor 38. Oxygen-containingreaction gas is supplied by means of feeding means 40 to the ash reactor38. When the solid material reacts with the reaction gas, the residualcarbon in the solid material either combusts to carbon dioxide CO₂ or itgasifies mainly to carbon monoxide CO. At the same time, the hydrocarboncompounds in the solid material, which are harmful to the environmentdecompose to a form, in which they are no longer harmful to theenvironment. Combustion of residual carbon generates heat energy andconverts the ash of the gasifier to a form, in which it may easily beutilized or collected. By gasifying the residual carbon it is possibleto increase the gas yield of the plant.

The ash reactor 38 may be, for example, a circulating fluidized bedgasifier or a bubbling bed gasifier. The reaction gas to be supplied tothe ash reactor 38 by means 40 fluidizes solid material bed forming inthe reactor, whereby the small ash particles of the bed entrain with theexhaust gas generated in the reactor through an outlet opening 42 of thereactor to a particle separator 44. The separation efficiency of theseparator has been chosen in such a manner that a sufficient amount ofash particles remain unseparated and are entrained with the exhaust gasthrough a conveying duct 46 to the product channel 28. Owing to the ashflow recirculated from the ash reactor 38, the ash content of theimpurities entrained with the product gas in the product gas channel 28increases considerably. This decreases sticking of the tar compoundscondensing in the gas cooler 30 on the surfaces.

According to a preferred embodiment of the present invention, ashparticles being guided to the product channel 28 from the ash reactor 38are cooled before being introduced to the product gas channel byutilizing a heat exchanger 54 arranged to the conveying duct 46. Thus,the tendency of the tar compounds in the product gas to condense on theash particles being returned from the ash reactor 38 increases andsticking of the tar compounds to the product gas channel 28, on the heatsurfaces of the gas cooler 30 and the filter 32 decreases.

Part of the particles separated by the separator 44 is returned to theash reactor 38 and part is discharged to an ash hopper 48 by means of acooled screw conveyor 50. The conveying velocity of the screw conveyor50 determines how much of the ash being separated by the separator 44 isdischarged from the system and how much is flown as overflow from adistribution chamber 52 back to the ash reactor 38. Due to thecombustion or partial combustion of the residual carbon of the solidmaterial, the temperature of the ash reactor 38 is preferably about650-950° C., for example, 850° C. Since the solid material beingreturned from the filter 32 is at a lower temperature than the ashreactor 38, it is possible to adjust the temperature of the reactor bychanging the amount of the ash recirculating through the ash reactor 38,gas cooler 30 and filter 32.

A second embodiment of the present invention disclosed in FIG. 2 differsfrom that of FIG. 1 in that in addition to filter ash, the ash reactor38 is also supplied with bottom ash of the gasification reactor 10pneumatically along a conveyor pipe 54 and material separated from thereturn duct 22 of the particle separator 20 by means of a screw conveyor56. Alternatively, it is possible to supply merely bottom ash of thegasification reactor or ash separated from the particle separator of thehot cycle or different combinations of the above mentioned ash flows tothe ash reactor 38. In the embodiment disclosed in FIG. 2 ash treated inthe ash reactor is pneumatically conveyed along a pipe 46′ from thebottom of the ash reactor 38 to the upper part of the gasificationreactor 10. Ash treated in the ash reactor may alternatively also beguided to the gas cooler 30 or elsewhere upstream of the gas cooler, forexample, to the product gas channel 28.

In an embodiment according to FIG. 2, the average particle size of theash being returned to the gasification system is bigger and the relativesurface area smaller than in the embodiment of FIG. 1. The advantage ofbigger particles is lesser tendency to stick on the surfaces of theproduct gas channel, so the embodiment of FIG. 2 is especiallyadvantageous when the product gas contains especially sticky tarcompounds, the amount of which is not very high.

While the invention has been herein described by way of example inconnection with what is presently considered to be the most preferredembodiments, it will be apparent to those of ordinary skill in the artthat many modifications and combinations may be made of the disclosedembodiments. Thus, the invention covers several other applicationsincluded within the scope of invention as defined in the appendedclaims.

1. A method of gasifying carbonaceous material, said method comprisingfollowing stages: a) gasifying carbonaceous material o product gas in agasification reactor of a gasification system; b) discharging productgas, sh entrained therewith, residual carbon and gasified tar compoundsfrom the gasification reactor to a product gas channel; c) coolingproduct gas discharged from the gasification reactor being cooled in agas cooler arranged in the product gas channel, whereby tar compoundsare condensed to a liquid form prone to stick on heat surfaces of thegas cooler; d) separating solid material containing ash particles andresidual carbon from the gasification system; and e) guiding solidmaterial separated from the gasification system to an ash reactor, andsupplying to which oxygen-containing gas to the ash reactor, whereby theresidual carbon of the solid material reacts with oxygen, and ashparticles and exhaust gas are generated; characterized in that saidmethod further comprises a stage of f) guiding ash particles from theash reactor along a conveying duct to the gas cooler or upstream of thegas cooler, whereby the ash content of the product gas increases and thesticking of the condensing tar compounds on the heat surfaces of the gascooler decreases.
 2. Method in accordance with claim 1, characterized inthat in stage f) ash particles are guided from the ash reactor to theupper part of the gasification reactor or to the product gas channel. 3.Method in accordance with claim 1, characterized in that in stage f) ashparticles are guided from the ash reactor to the product gas channel. 4.Method in accordance with claim 1, characterized in that in stage d) ashparticles and solid material containing residual carbon are separatedfrom the product gas.
 5. Method in accordance with claim 1,characterized in that the ash reactor is a fluidized bed reactor and instage f) exhaust gas from the ash reactor and ash particles entrainedtherewith are guided to the gas cooler or upstream of the gas cooler. 6.Method in accordance with claim 5, characterized in that the coarsestpart of the ash particles entrained with the exhaust gas of the ashreactor is separated from the exhaust gas and in stage f) the rest ofthe ash particles entrained with the exhaust gas of the ash reactor isguided with the exhaust gas to the gas cooler or upstream of the gascooler.
 7. Method in accordance with claim 1, characterized in that thetemperature of the ash reactor is adjusted by controlling the amount ofash to be guided to the gas cooler or upstream of the gas cooler. 8.Method in accordance with claim 1, characterized in that in stage f) ashparticles are cooled in the conveying duct.
 9. Method in accordance withclaim 1, characterized in that in stage e) residual carbon in the solidmaterial is combusted in the ash reactor.
 10. An apparatus for gasifyingcarbonaceous material, said apparatus comprising a gasification system,including a gasification reactor, a product gas channel connected with agasification reactor, a gas cooler arranged in said product gas channeland means for separating solid material containing residual carbon fromthe gasification system, an ash reactor, having means for treatingresidual carbon from ash with oxygen; and means for guiding solidmaterial separated from the gasification system to the ash reactor,characterized in that the apparatus comprises means for guiding ashparticles treated in the ash reactor from the ash reactor to the gascooler or upstream of the gas cooler.
 11. Apparatus in accordance withclaim 10, characterized in that the apparatus comprises means forguiding ash particles treated in the ash reactor from the ash reactor tothe upper part of the gasification reactor or means for guiding the ashparticles treated in the ash reactor from the ash reactor to the productgas channel.
 12. Apparatus in accordance with claim 10, characterized inthat the apparatus comprises means for guiding ash particles treated inthe ash reactor from the ash reactor to the product gas channel. 13.Apparatus in accordance with claim 10, characterized in that means forseparating solid material containing ash particles and residual carbonfrom the gasification system comprise a particle filter arranged in theproduct gas channel.
 14. Apparatus in accordance with claim 10,characterized in that the gasification reactor is a fluidized bedgasifier.
 15. Apparatus in accordance with claim 14, characterized inthat the gasification reactor is a circulating fluidized bed reactor.16. Apparatus in accordance with claim 10, characterized in that the ashreactor is a fluidized bed reactor.
 17. Apparatus in accordance withclaim 16, characterized in that the ash reactor is a circulatingfluidized bed reactor.
 18. Apparatus in accordance with claim 16,characterized in that the means for guiding ash particles treated in theash reactor from the ash reactor to the gas cooler or upstream of thegas cooler comprise means for guiding exhaust gas of the ash reactor andash particles entrained therewith from the ash reactor to the gas cooleror upstream of the gas cooler.
 19. Apparatus in accordance with claim18, characterized in that means for guiding the exhaust gas of the ashreactor and ash particles entrained therewith from the ash reactor tothe gas cooler or upstream of the gas cooler comprise means forseparating the coarsest portion of the ash particles from the exhaustgas.
 20. Apparatus in accordance with claim 19, characterized in thatmeans for separating the coarsest portion of the ash particles from theexhaust gas are provided with means for removing a first portion of theseparated coarsest part to an ash hopper and for returning a secondportion to the ash reactor.
 21. Apparatus in accordance with claim 10,characterized in that means for guiding ash particles from the ashreactor to the gas cooler or upstream of the gas cooler are providedwith means for cooling ash particles.
 22. Apparatus in accordance withclaim 10, characterized in that the ash reactor is an ash combustionplant.